The roots of most varieties become tough and woody as plants grow larger, so it's best to make small plantings at two-week intervals for a continuous supply of tender, crisp bulbs. Although more heat-tolerant than other members of the cabbage family, kohlrabi grows best in cooler weather. Heat and dry soil may result in tough, woody bulbs. In areas with hot, dry summers, it's best to sow in spring for harvest in June or in late summer for harvest in October. In most of Canada, however, kohlrabi can be grown throughout the season. Gardeners in coastal British Columbia can plant 'Superschmeltz'—a large variety—in late summer and harvest it until April.
Kohlrabi needs consistent moisture. Soil that's alternately dry and wet causes bulbs to become woody or split. Mulch young plants with hay, straw, grass clippings or composted leaves. In a dry summer, regularly check under the mulch to make sure soil is moist. Every four weeks, work some compost into the soil around the base of the plant, being careful not to disturb the roots.
Most varieties are best harvested when bulbs are four to five centimetres in diameter. Harvest by pulling up the entire plant or by cutting 2.5 centimetres beneath the bulb, leaving the root behind. For immediate use, snap off leaf stems then peel off the skin and any fibrous areas. Very small, tender bulbs can be eaten with the skin intact.
It's better to harvest and store bulbs than to leave them in the ground. Remove the leaves but not the skins from bulbs to be stored. Small bulbs keep in the refrigerator for two to three weeks, larger bulbs for up to three months in a refrigerator or root cellar. In areas with cold winters, varieties with large bulbs can be left in the ground, covered with deep mulch-up to one metre in cold areas-and harvested throughout the winter.
To freeze kohlrabi, wash, trim and skin the roots, then cut into one-centimetre-square pieces before blanching and freezing them.
Pests and diseases
Kohlrabi is generally less plagued by diseases and pests than other members of the cabbage family. To avoid potential problems, keep your garden well weeded, remove and destroy any plants infected with disease and do a good fall cleanup. Practise crop rotation - members of the cabbage family should not be replanted in the same place for three to four years.
Aphids Pinhead-sized insects with pear-shaped bodies that suck juices from plants, causing leaves to curl and turn yellow; plants may stop growing. Wash off aphids with a shower of water. For serious infestations, spray twice a week with insecticidal soap.
Flea beetles Shiny, two-millimetre-long beetles that chew tiny holes in leaves, killing young seedlings, and spread disease. Cover seeded rows and recent transplants with floating row covers. As a last resort, spray with rotenone.